Increasing Chloride Concentrations in Shallow Groundwater and Surface Water in the Chicago Region

Population and infrastructure are growing rapidly in the Chicago, Illinois, metropolitan area. Population is projected to increase from 8 million in 2005 to more than 10 million by 2030. Most of the growth is occurring in collar counties such as Kane, McHenry, and Will, where populations may double in that period. Demand for water is also increasing substantially. Shallow bedrock and overlying sand and gravel aquifers, which can be vulnerable to surface contaminants, are expected to be the main sources of water to meet the increased demand in the Chicago region. A statistical study of historical water quality data for shallow aquifers (textless 200 ft) has indicated that chloride (Cl - ) concentrations have been increasing since the 1960s, particularly in collar counties west and south of Chicago. The primary source of Cl - is road salt runoff. About 43 concentrations greater than 1 mg/L/yr and 15 mg/L/yr. Approximately 24990s had Cl - concentrations greater than 100 mg/L (35; median values were less than 10 mg/L prior to 1960. The greater increase in Cl - concentrations in the collar counties is likely due to both natural and anthropogenic factors, including the presence of more significant and shallower sand and gravel deposits and less curbing of major highways and streets. Elevated levels of Cl - have also been observed in the Illinois River waterway. In a recent study, samples were collected from the river and selected tributaries from Chicago to its confluence with the Mississippi River over a two-year period. Chloride concentrations in the Illinois River ranged from 40 to 488 mg/L and spiked during the late winter and early spring as a result of road salt runoff. A large component of Cl- throughout the year was attributed to treated wastewater (TWW) from the Chicago area, and during periods of low flow, elevated Cl - levels attributable to TWW were detected all the way to the confluence with the Mississippi River. Chloride concentrations in the Illinois River at Peoria have been increasing since the 1950s; the annual median increased from about 20 mg/L in 1946 to near 100 mg/L in 2005, approximately 1.0 mg/L/yr. The greatest concentrations and rates of increase occur between January and March, due to road salt runoff